feat(configure): auto-detect project_id

[tvt286]

Summary

Follow-up to #13. When the user leaves the Project ID prompt blank, grn configure now calls vServer /v1/projects with the just-entered credentials and region's vServer endpoint, then saves the returned project_id.

Each user has exactly one project per region (confirmed), so the first result is unambiguous.

Flow

Client ID [****abcd]:
Client Secret [****wxyz]:
Default region name [HCM-3]:
Default output format [json]:
Project ID (leave blank to auto-detect) []: <press Enter>
Fetching project_id from HCM-3...
Auto-detected project_id: pro-e28d4501-59d8-4f47-9eb0-948c54f80592
Configuration saved successfully.

Failure handling

• Bad credentials → warning, saves blank
• Network error → warning, saves blank
• No project → warning, saves blank

In all cases the wizard completes successfully; downstream tools (greenode-mcp-server) can still auto-detect at call time.

Changes

• go/cmd/configure/detect_project.go: new helper — detectProjectID() and vserverEndpointForRegion()
• go/cmd/configure/configure.go: invoke auto-detect when projectID is empty
• README.md + docs/configuration.md: updated wizard output example
• .changes/next-release/enhancement-configure-*.json: changelog fragment

Test plan

• go vet + go build clean
• Manual: run wizard with valid credentials → project_id auto-populated
• Manual: run wizard with bad credentials → warning, saves blank
• Manual: run wizard with explicit project_id entered → auto-detect skipped
• grn configure list shows the saved value

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